1. Field of the Invention
This invention relates to a method of centering and thus balancing workpieces to be machined only in part, particularly crankshafts, which method comprises a provision of centering bores in end faces of the workpiece and/or a circular machining of peripheral surfaces on which the workpiece is adapted to be gripped, wherein the workpiece before it is machined is repositioned so that the rotary unbalance of the workpiece is reduced in that the position and/or mass distribution of the workpiece relative to its axis of rotation is changed, this is optionally succeeded by a cutting of the workpiece to a predetermined length, the workpiece is subsequently provided with the centering bores and/or with the surfaces to be gripped, which define the balanced and centered position of the workpiece and the associated axis of rotation, and the magnitude and direction of the change of the position and/or mass distribution of the workpiece are determined in consideration only of the rotary unbalances which are due to those portions of the workpiece which are intended not to be machined.
2. Description of the Prior Art
Such a balancing centering has already been proposed in EP-81-0 268 724 and permits a satisfactory balancing of the workpiece to be effected in an economical manner in the course of the manufacture of the workpiece. Because the rotary unbalances which are due to those portions of the workpiece which are to be machined are not taken into account, the axis of rotation which is determined by the centering bores or the surfaces to be gripped will provide after the balancing operation for a rotary unbalance only of those portions of the workpiece which are not intended to be machined so that the final balancing operation which will succeed the machining operation has been preceded by an optimum preliminary balancing operation because the machining of the workpiece will be performed with reference to the axis of rotation which has been determined by the centering operation and will eliminate those rotary unbalances which are due to the those portions of the workpieces which have been machined and, as a result, have a mass distribution which is balanced with reference to the axis of rotation. The final balancing operation may then be restricted to a precision balancing, which can economically be performed and even in workpieces having unmachined portions will ensure a perfectly balanced true running about the axis of rotation which has been established.
To permit a determination of the magnitude and direction of the displacement of the workpiece in the previous practice, the external configurations of those portions of the workpieces which were intended not to be unmachined and were measured and were compared by a computer with the desired shapes of the corresponding portions of an ideal workpiece. But it is rather difficult and complicated to measure the unmachined portions of the workpiece with the required accuracy and this can be accomplished only by means of special measuring instruments, which are highly expensive.